Embodiments relate to signaling reference signals in a wireless network, for example, a Long Term Evolution (LTE) wireless network.
The 3GPP Long Term Evolution (LTE) represents a major advance in cellular technology. LTE is designed to meet carrier needs for high-speed data and media transport as well as high-capacity voice support well into the next decade. LTE encompasses high-speed data, multimedia unicast and multimedia broadcast services.
The LTE physical layer (PHY) is an efficient mechanism for conveying both data and control information between an enhanced base station (eNodeB) and mobile user equipment (UE). The LTE PHY employs some advanced technologies to cellular applications. These technologies include Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission. In addition, the LTE PHY uses Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink (DL) and Single Carrier-Frequency Division Multiple Access (SC-FDMA) on the uplink (UL). OFDMA allows data to be directed to or from multiple users on a subcarrier-by-subcarrier basis for a specified number of symbol periods.
Existing carriers in LTE systems include Cell-Specific Reference Signals (CRS) in certain Resource Elements (REs) in every subframe.
In a recent LTE release, new types of carriers are to be introduced, one aim of which is to reduce overhead from non-data-bearing signals such as CRS. However, simply to omit the CRS is not necessarily possible, because the CRS may continue to be required for the UEs to make measurements (e.g. for monitoring of the serving radio link or for measuring interference in neighboring cells) and/or for the UEs to maintain synchronization.
Similar issues exist with the Channel State Information Reference Signals (CSI-RS). Although the CSI-RS are not transmitted in every subframe (unlike the CRS), they are nonetheless transmitted using the full system bandwidth and with a periodicity which is configured semi-statically (i.e. cannot be modified dynamically). Reducing the bandwidth of the reference signals would also have the advantage of enabling the transmission bandwidth of the carrier to be better matched to the bandwidth that may be available.
Solutions have been proposed where the CRS and/or CSI-RS may be transmitted only in certain resource blocks or only in certain subframes. The solution is to signal a bandwidth over which UEs are expected to make measurements (known as the “set S” subbands). However, the details of the signaling have not been proposed. Moreover, signaling the subbands for measurements is not sufficient, because even UEs which have no measurements configured are required to know in which PRBs the reference signals are present, for synchronization/tracking purposes.
A further shortcoming of the aforementioned solution is that the CSI-RS are configured with a periodicity and a time offset. However, simply reconfiguring the periodicity may not provide sufficient flexibility. For example, short regular bursts of a few subframes in which reference signals are provided, with no reference signals in the subframes between the bursts, may be a useful configuration for making measurements.
A fully flexible approach, with a complete 2-dimensional bit-map of, for example, 110 RBs in the frequency domain and 40 subframes in the time domain, thus including 4400 bits, is impractical and itself constitutes a higher overhead which would make it prohibitive to adapt the reference signal pattern.
Example embodiments provide a method and apparatus to reduce overhead but do not remove required reference signals.